This invention relates generally to a method of evaluating gravel pack slurry (including a resultant gravel pack) and more particularly, but not by way of limitation, to a method of determining one or more characteristics about a gravel pack slurry under dynamic conditions wherein the slurry is flowing relative to one or more fast neutron sources and thermal neutron detectors.
In some formations into which a well has been drilled for producing oil or gas, sand will be produced along with the oil or gas. This is not desirable because sand can cause problems such as equipment damage and reduced production of the oil or gas. One way to inhibit sand production is to pump gravel (i.e., typically larger grained sand) down into the well so that it packs tightly in the annular space between a screen and the formation (or casing cemented to the formation) to minimize the movement of sand grains produced from the formation during the production of oil or gas. The effectiveness of this treatment can be critical to the viability of the well; therefore, proper planning and execution of a gravel pack job are important.
To properly plan at least some types of gravel pack jobs, tests of different fluids should be performed in a laboratory to determine which fluid appears to be best for the particular well environment. These fluids are typically slurries of gravel mixed in a carrier liquid containing various constituents known in the art (although "sand" is commonly the packing component of such a slurry, the term "gravel" will be used herein to be consistent with the terminology "gravel pack" and to be distinguished from the "sand" which flows out of the formation with the oil or gas and which is to be blocked by the gravel pack).
Heretofore, gravel pack slurries have typically been tested in a large physical model or via computer modeling. The former is expensive and does not produce all the information desired, and the latter does not yield direct results of what is actually happening in a slurry. One particular shortcoming of the physical model is that the void spaces in the gravel pack cannot be determined unless the model is made with a transparent material, such as plastic or glass; but such a transparent model can have temperature and pressure limitations precluding simulations at actual elevated downhole temperatures and pressures. Even if the void spaces could be seen, there would not be a quantitative analysis of the gravel pack. Where the void spaces are, and their quantification, are important information because a successful gravel pack depends on the percentage of void space which exists in the pack. That is, the less void space there is, the less chance there is for sand to be produced with the oil or gas. Furthermore, physical models that derive evaluation data from pressure transducer measurements can have limited spatial resolution and accuracy.
Determining whether a planned gravel pack job is being successfully performed has been even more elusive than properly planning a gravel pack job. That is, we are not aware of a method which monitors what actually happens as a gravel pack slurry is pumped into a well and which provides data for indicating characteristics of the actual gravel pack formed (e.g., packing efficiency, gravel concentration, porosity, density, patterns of gravel packed in the well, and gravel settling rate).
In view of the aforementioned shortcomings, there is the need for a method for evaluating a gravel pack slurry, whether in a laboratory test environment or down in an actual well environment. Such a method should directly test the slurry in that it should provide direct responses to an actual slurry in its displacement in a pipe or annulus. For enhanced resolution, it should be capable of evaluating multiple discrete volumes of the overall volume of slurry. More particularly, the method should be capable of providing information from which packing efficiency, gravel concentration, porosity, density, and patterns of gravel packed in the well, and gravel settling rate can be determined.